236 ABSORPTION-CELLS ON LEAVES. 
situations erystals of salt are sometimes to be seen separated out from the soil, and 
lying as a white efflorescence upon the ground, it used formerly to be believed that 
the salt inerusting leaves and stems was derived, not from the plants in question, 
but from the soil around, and had only spread from there over the various plant- 
members. But this is not the case. As a matter of fact, the salt observed on the 
leaves and stems of Frankenia, Reawmuria, Hypericopsis persica, and a few species 
of Tamaria and Statice,is produced from the substance of the leaves. It is excreted 
in just the same way as the crust of lime, above described, is from the leaves of 
saxifrages. To the naked eye the surfaces of the leaves in all the plants enumerated 
have a punctate appearance. On closer inspection, it is evident that, corresponding 
to each dot, there is a little cavity, the deepest part of which is constructed of cells 
with extremely delicate external walls. In quite young leaves only a single thin- 
walled cell of the kind is to be seen at the bottom of each shallow depression. But 
this divides, and, by the time the leaf is full-grown, from two to four cells are seen 
to have arisen by division of the one cell. Stomata are, in addition, intercalated in 
the membrane in the neighbourhood of these thin-walled cells, and, in the rainy 
season, when there is no lack of water in the habitats of the plants in question, a 
watery juice, containing a large amount of salts in solution, exudes from these 
stomata. The saline solution soaks over the whole surface of the leaf, and in a dry 
atmosphere crystals form from it and adhere to the leaf in the form of little gland- 
like patches or continuous crusts. 
If these tamarisks, frankenias, and reaumurias are observed during a rainless 
season, the crystals of salt are seen under the noon-day sun glittering on the leaves 
and stems, and may be detached in the form of a fine crystalline powder. But if 
the same place is visited after a clear night, no trace of crystals is to be seen; the 
little leaflets have a green appearance, but they are covered with a liquid with a 
bitter salt taste! and are damp and greasy to the touch. The crystals have 
attracted moisture from the air during the night, and have deliquesced, and the 
saline solution not only covers the whole of the leaf, but also fills the little cavities 
visible as dots to the naked eye. The thin-walled cells at the bottom of the cavi- 
ties differ from the rest of the epidermal cells and the guard-cells of the stomata, in. 
that they are susceptible of being wetted, and they may act as absorption-cells, and 
allow the water, attracted by the salts from the air, to pass through their thin 
walls into the interior of the leaves. 
When the air dries under the rising sun, erystals are again formed from the 
solution of salts, and, covering the leaves once more in the form of crusts, fill up the 
depressions and protect the plants during the hot hours of the day from excessive 
evaporation. Whilst, therefore, in the dewy night these plants are indebted to 
their salt crusts for water, they are in the day-time preserved from desiccation by 
the action of the same contrivance. 
1 The salt incrustations which were removed from plants of Frankenia hispida, collected on a Persian salt-steppe, 
consisted principally of common salt (chloride of sodium). They contained in smaller quantities, gypsum, mag- 
nesium sulphate, calcium chloride, and magnesium chloride. 
